In a turbomachine of the above-specified kind, a compressor stator is fitted with at least one stage of stator vanes that are variable in pitch, with the orientation of the vanes in the flow section being adjustable. Thus, the angle of attack of the vanes can be controlled as a function of operating conditions by a servo-control system that causes said actuator ring to move.
By way of example, U.S. Pat. No. 4,130,375 describes an actuator ring that moves circularly only, the ring being made up of two superposed rings. The inner ring constitutes a track ring mounted stationary on the casing by means of radially-extending studs, and the outer ring is the actuator ring proper, being attached via links to vane-actuation cranks, and running on the track ring via a system of wheels. Such a system is heavy and bulky, other things being equal.
In addition, the actuator ring is driven by one or more actuators. As a result the accuracy with which the vanes are positioned and the speed with which they are operated can be insufficient.
Furthermore, European patent No. 0 527 593 describes a linear electric motor structure adapted to driving an actuator ring. The system described makes use of electrical induction elements situated radially outside said actuator ring. The induction elements are bulky and difficult to integrate in the space available outside the casing. In addition, the actuator ring is not centered in satisfactory manner, particularly when considering possible variations in diameter (due to the temperature rise) between the actuator ring and the casing that supports it.